Despite decades of research on Escherichia coli and Salmonella typhimurium, microbiologists are still unable to assign, or even convincingly predict, functions for more than 30 percent of the open reading frames (ORFs) in the E. coli genome sequence. It seems likely that the functions of many of these genes may not be observable using pure cultures. In nature, such bacteria do not normally exist as pure cultures and a percentage of their genetic capacity is almost certainly involved with 'mixed community' interactions. Consistent with this hypothesis, we recently identified an S. typhimurium receptor that is activated only within a mixed microbial community. The signaling event discovered is the detection of N-acylhomoserine lactones (AHLs) by SdiA, a LuxR homolog found in S. typhimurium. The sdiA gene is also present in E. coli and Klebsiella spp, suggesting that it has a relatively ancient function. However, what is truly remarkable is that these species do not produce the ligands that are detected by SdiA. Instead, SdiA detects uncharacterized compounds present in mammalian intestines and AHLs produced by other species of bacteria. We will test the hypothesis that the intestinal compounds are microbial in origin and attempt to identify the species producing these molecules. We will use microarrays to identify the portions of the E. coli and S. typhimurium genomes that are regulated by SdiA in response to AHLs and characterize the phenotypic consequences of this signaling event.